LNG (liquefied natural gas) generally refers to colorless, transparent cryogenic liquid converted from natural gas (predominantly methane) that is cooled to approximately −163° C. and condensed to 1/600th the volume.
As LNG emerges as an energy source, efficient transportation means have been sought in order to transport LNG from a supply site to a demand site in a large scale so as to utilize LNG as energy. Resulted in a part of this effort is LNG carriers, which can transport a large quantity of LNG by sea.
LNG carriers need to be furnished with a cargo that can keep and store cryogenically liquefied LNG, but such carriers require intricate and difficult conditions. That is, since LNG has vapor pressure that is higher than atmospheric pressure and boiling point of approximately −163° C., the cargo that stores LNG needs to be constructed with materials that can withstand very low temperature, for example, aluminum steel, stainless steel and 33% nickel steel, and designed in a unique insulation structure that can withstand thermal stress and thermal contraction and can be protected from heat leakage, in order to keep and store LNG safely.
Particularly, membranes, which are the primary barrier of the cargo, are in direct contact with the cryogenic LNG with its temperature of −163° C., and thus are made of metallic materials, such as aluminum alloy, the Invar, 9% nickel steel, etc., which are strong against brittleness at a low temperature and can address changes in stress. Membranes also have linear corrugations, in which the center is bulged, in order to allow easier expansion and contraction in response to repeated changes in temperature and change in the weight of the stored liquid. In addition, membranes have weld zones that help keep the tank leak-proof by fold-welding edges of a plurality of membrane panels.
In the conventionally-used membranes, the membranes are made in an approximately rectangular shape, and a plurality of corrugations are formed throughout the membrane panels in order to facilitate expansion and contraction in response to heat and load. Moreover, corners and 4 sides of a single membrane panel, which encompasses the plurality of corrugations, are overlapped and connected by welding with corners and 4 sides of neighboring membrane panels to make the tank leak-proof.
However, since the corrugations of the conventional membranes are bulged, the membranes are expected to collapse easily under increased hydrostatic or dynamic pressure in the cargo as LNG carriers become increasingly bigger. For example, the hydrostatic pressure applied by liquefied gas may cause considerable plastic deformation of the corrugations, and particularly, lateral faces of the corrugations that are at a certain distance away from intersecting corrugations may be crushed.
There have been a number of efforts to reinforce the rigidity of the corrugations, for example, increasing the thickness of the membrane, but these efforts have had problems such as decreased flexibility. As illustrated in FIG. 1 and FIG. 2, US2005/0082297 discloses a sealed wall structure including at least one membrane 10, in which a series of first corrugations 5 and a series of second corrugations 6, the directions of which are perpendicular, are formed in the membrane, in which the corrugations 5, 6 protrude toward an internal face of a tank, in which the sealed wall structure includes at least one reinforcing ridge 11 formed on at least one corrugation midway between two intersections 8 with the other series of corrugations, and in which each ridge 11 is generally convex and is locally formed on at least one lateral face of the corrugation supporting the ridge.
However, as illustrated in FIG. 2, the corrugations, the facial rigidity of which is increased by the reinforcing ridge, of the conventional membrane described above may not properly function to expand and contract as expected when force is exerted on the corrugation in the direction of the arrow, thereby increasing the stress in the weld zones during thermal contraction. Moreover, since the parts that do not receive pressure or receive little pressure do not need the reinforcing ridge, membranes with reinforcing ridges and membranes without reinforcing ridges both need to be provided and arranged properly during the construction.